Terminal for low-temperature cable

ABSTRACT

A terminal for a low-temperature cable having a plurality of coaxial ducts defining annular channels through which a cryogenic fluid is passed and a cable conductor extending through the inner duct, has a generally tubular terminal conductor traversed by a coolant which is fed or received from the coolant chamber of the duct and passes through a single inlet or outlet conduit, at the terminal. The conductor traverses a low-temperature (deepcooling) zone, a cooling zone and a warm zone from the duct to the terminal end and may be connected externally of the terminal to an electrical supply source or a load.

United States Patent 1191 Hildebrandt 1451 May 27, 1975 [54] TERMINAL FOR LOW-TEMPERATURE 3,522,361 7/1970 Kafka 174/15 C CABLE 3,725,565 4/1973 Schmidt 174/15 c x 3,728,463 4/1973 Kullman et a1 174/15 C X Inventor: gllrich Hildebrandt, Munich, 3,743,760 7/1973 Sassin 174/15 ermany FOREIGN PATENTS OR APPLICATIONS 1 1 Assignee: Linde Aktiengesellschaft, 1,548,640 /1968 France 174/D1G. 6 wsbad Germany 2,004,252 11/1969 France 174/DIG. 6

22 Pl d: 0 t. 1973 1 I e c a Primary ExaminerArthur T. Grimley [21] Appl. No.: 404,681 Attorney, Agent, or Firm-Karl F. Ross; Herbert Dubno Foe A l't'oP"tDt 1 r ign pp M11 11 non y a a ABSTRACT Oct. 10, 1972 Germany 2249560 A termmal for a low-temperature cable having a plu- 52 U.S. c1. 174/15 11:; 174/15 BI-l; 174/19; ramy of cPaxial ducts, f i annular channels 174/DIG 6 through WhlCl'l a cryogenic flu1d is passed and a cable 51 1m. 01 H01v 11/00 condumr extending through the inner duct has a [58] Field of Search H 174/l1 BH, 12 BH, 15 BH, generally tubular terminal conductor traversed by a l74/15 C, 15 R 13, 16 R, 19, DIG 6; 335/216 coolant WhlCl'l 1s fed or received from the coolant chamber of the duct and passes through a single lnlet [56] References Cited or outletlcontduit, atthe te'minal. "lihe ;:onductor traverses a owempera ure eep-coo mg zone, a coo UNITED STATES PATENTS ing zone and a warm zone from the duct to the termi- 650,987 6/1900 Ostergren l74/DIG. 6 end d may b connected externally f th t inal to an electrical supply source or a load. 3:43lz347 3/1969 Kafka et a1 174/15 C 23 Claims, 3 Drawing Figures LIQUID NITROGEN VA/CUUM HELIUM 52' 5f 54 i 5/, 54 5e 32 5 9 292 5 #94 '96 5 5 VACUUMC 26 6 I A? 24 a la 6 fl 26. 2 12 5201. H ,6 3 7 /0 a 35/, t 4 4 6 H20 6 Z? 7 2 20 6 4 L L437 1 5 Z5; f@' K22 E ATENTED MAY 2 71975 SHEET TERMINAL FOR LOW-TEMPERATURE CABLE I FIELD OF THE INVENTION The present invention relates to a terminal for a lowtemperature cable and, more particularly to a terminus for a cable-conduit system in which an electrical conductor extendsjthrough a duct arrangement cooled by a cryogenic fluid.

BACKGROUND OF THE INVENTION The principle of decreasing the electrical resistivity or increasing the electrical conductivity of a conductor by cooling it with a cryogenic fluid, e.g. a refrigerant circulated in a refrigeration cycle or a liquefied gas, has been used increasingly in transmission lines of high capacity.

In an extreme case, the cryogenic fluid lowers the temperature of the conductor to its superconductive state thereby rendering its resistance negligible and I permitting the conductor to carry large currents.

In one such system a superconductive coaxial cable is provided with three mutually coaxial ducts through one or more of which the cryogenic fluid or fluids may be passed. A terminal for this arrangement may be provided at each end of the duct system and can include a contract member at which electrical connection is made to the conductor traversing the innermost duct.

Because the terminal must provide low heat flow into thev system, perfect sealing of the several ducts, electrical separation of the current-carrying conductor from ground and negligible electrical and fluid leakage, the conventional design has been complex and its manufacturing and material costs have been inordinately high.

OBJECT OF THE INVENTION It is the principal object of the present invention to provide a terminal for a low-temperature cable which is relatively simple to produce and which affords the requisite security with respect to electrical insulation, termal loss and fluid tightness.

Another object of the invention is to provide an improved terminal for a low-temperature cable whereby the disadvantages described above of the earlier systems are obviated.

SUMMARY OF THE INVENTION These objects, and other which will become apparent hereinafter are attained in accordance with the present invention with a terminal or end structure for a lowtemperature cable consisting of a coaxial duct system having an electrical conductor extending through the innermost duct and defining channels at least one of which is traversed by at least one cryogenic fluid as described above, the cable comprising a thermally insulated central duct (through which the electrical conductor extends) and a plurality of coaxial corrugated outer ducts defining the annular channels around the inner duct, the terminal being so constructed and arranged that the conductor passes successively through a warm zone, a cooling or cool zone and a lowtemperature or deep-cooling zone axially inwardly from the terminal and toward the main portion of the duct system or cable.

Advantageously, the duct defines coaxial chambers of decreasing temperature inwardly, corresponding to the three zones. The warm zone may correspond to a region maintained at ambient temperature or some degrees below ambient whereas the cool or cooling zone may be at a temperature from tens to hundred of degrees below ambient, while the low-temperature or deep-cooling zone is at cryogenic temperature (e.g. the temperature of liquefied helium) and, preferably, superconductive temperatures.

According to the invention, more specifically, the electrical conductor in the cooling zone comprises one or more parallel-connected tubes whose walls and/or inserts are electricallyconductive and which define a flow cross-section for a coolant and which, at its side turned toward the low-temperature deep-cooling zone is electrically connected to the conductor of the cable.

The electrical insulation in the low-temperature or deep-cooling zone, i.e., the separation from ground potential, is formed by an axially extending annular chamber containing cryogenic fluid. The chamber communicates on one side with the coolant (refrigerant) duct held at ground potential and at the opposite side with the interior or the central duct of the cable and the aforementioned flow cross-section of the tubular conductor in the cooling zone.

Such a terminal enables simultaneously the cooling of the electric conductor in the cooling zone and the electrical conductor of the cable which terminates at the low-temperature or deep-cooling zones and which extends through the central duct of the cable, using a single input conduit to supply or remove the refrigerant.

The refrigerant thus flow, at one end, from this conduit first through the axially extending annular chamber of the low-temperature or deep-cooling zone (which provides the electrical insulation) and is then distributed in part to the interior of the inner conduit of the cable (traversed by the cable conductor) and inpart to the flow cross-section of the conductive tube in the cooling zone.

In the other end of the cable, of course, the coolant flows from the central cable duct into the conductor tube of a similar terminal through the corresponding annular axially extending chamber of its lowtemperature or deep-cooling zone and out of the single duct (discharge duct) in the opposite direction.

For thermal insulation in common for the lowtemperature (deep-cooling) and cooling zones, a common annular vacuum compartment is coaxially provided and is equipped with a cooled radiation shield in the form of a sleeve.

Preferably, the radiation shield is cooled by thermal conduction, e.g. by connecting it mechanically in heatconducting relation to a radiation shield of the cable cooled by a refrigerant such as liquid nitrogen.

Within the evacuated or vacuum compartment an additional insulation, e.g. a multi-layer sheath, can be disposed. The sheath may consist of alternating layers of low-thermal-conductivity non-woven or woven fabric webs (e.g. of asbestos or glass fibers or synthetic resin filaments) strongly reflecting metal foils (e.g. of aluminum). This arrangement enables effective thermal insulation of the cooling zones of the terminals, one of which is provided at each end of the cable, without additional refrigerant compartments, i.e. without additional nitrogen-filled spaces.

Best results are obtained with the invention when the electrical conductor in the cool zone is composed of material whose specific resistance in the region of the operating temperature (between 4K and 300K) increases generally with T" where T is the absolute temperature and n is greater than 0.5.

Such a characteristic is attained with high-purity metals, e.g. high purity copper of 99.5% or greater purity, in which or onto which superconductive materials may be applied if desired.

By choosing a material with these characteristics, it is possible to achieve the desired distribution of the coolant along the ducts of the cooling zone. If the temperature of one of the conductors is to be lowered for example, the fraction of the coolant flow in contact therewith can be increased, thereby lowering the electrical resistance where increased electrical-heat generation may occur, thus maintaining the cooling-pressure drop and the voltage drop (per unit length) constant along the length of the conductor. The ability of the increased coolant-flow mass to pick up the heat can be matched to the heating effect of the conductor only when the heating effect, at least at declining temperatures of the conductor and therefore the coolant, increases with the coolant-flow mass. I have found that this requires, for turbulent flow, a value of n greater than 0.5.

Advantageously, the ducts of the cool zone, which at their cold ends are soldered on to an electrical conductive tube sheet or plate, are surrounded by a common shell which is either completely or partially formed from an electrically insulating material, or at least at some point is insulated from the electrically conductive ducts. Thus an electrical-current flow through the uncooled shell which is not traversed by coolant, and an electrical heating thereof does not occur.

At the warm end of the cool zone, the ducts are soldered directly into bores in the end faces of the thick-walled copper pipes of the warm zone and have inner spaces communicating by radial bores in the thick-walled pipes with the interiors of the latter so that, on the one hand, a good electrical connection is made between the pipes of the warm zone and the ducts of the cool zone and, on the other hand, the passage of the warm coolant from the cool zone is permitted.

To avoid thermal stresses within the shell surrounding the ducts of the cool zone, an elastic bellows is provided to seal the shell against the tube sheet at the cold side of the cool zone. The space defined by the elastic bellows and under coolant pressure is connected at the cold side of the coolant zone with the coolant space and constitutes a cold buffer or reservoir. With pressure fluctuations in the coolant, therefore, no unnecessary thermodynamic losses ensue since neither contact of warm coolant with cold parts not of cold coolant with warm parts can occur.

Further advantage is to be found in that the intervening space between the ducts of the cool zone is either evacuated or filled with a coolant at constant pressure. In this manner coolant in the intervening space does not flow between zones of different temperatures upon the development of pressure fluctuations, and thermodynamic losses are here eliminated as well. As part of the electrical insulation of the terminals, I provide a pressure-resistant synthetic-resin tube or sleeve which surrounds each duct extending from the coolant part of the deep-cooling or low-temperature zone through the cool zone and into the warm zone of the terminal.

The synthetic-resin tubes or sleeves are sealed relative to the ducts which they surround in the warm zone.

To improve the electrical insulation characteristics in the deep-cooling or low-temperature zone, within the axially extending annular channel formed between each pressure-retaining synthetic-resin tube or sleeve and the reinforced conductor insulation, I provide helical electrically insulating spacers which define a helical or screw-like flow path for the coolant. In addition, or alternatively, the two walls defining these annular channels may be formed with steps similar to those of open-air insulators.

Within the warm zone, the space between the conductor and an outer porcelain insulator, into which the pressure resistant synthetic-resin sleeve extends, is filled with oil having a low permissible specific conductivity.

According to another feature of this invention, the common vacuum shell and the central channels of the deep-cooling or low-temperature zone and the cool zone are sealed with respect to one another, with respect to the atmosphere and with respect to the oilfilled space by an improved sealing arrangement described hereafter.

The improved sealing arrangement comprises a flange junction in which one flange is provided at the region of the warm end of the cool zone on the periphery of the central-channel duct and the other flange and which is disposed parallel to the first and is connected to other upper end of the vacuum shell and the lower end of the porcelain insulator. The flange junction includes two seals, one between two interconnected flanges and the other on the pressure-resistant synthetic-resin sleeve.

Between both seals there is provided an annular canal which, via a venting passage, is preferably connected with the atmosphere or is connected with the oil-filled space of the warm zone. Thus one seal is effective to seal the vacuum space against the annular canal while the other seal is effective to seal off the central channel, the annular canal and the oil-filled space with respect to one another.

With this construction, the danger is avoided of failure of one of the seals allowing direct passage of coolant into the vacuum spaces. Furthermore, leaks within the terminal are readily detected by monitoring the vent passage.

According to still another feature of this invention, the various vacuum spaces are insluated from one another, i.e., the vacuum space of the cable is isolated from the vacuum space of the terminals. For this purpose the axially spaced ends of the corresponding corrugated ducts are provided with force-transmitting and sealingly fitted flanges between each pair of which an elastic low-thermal conductivity gas-type bellows is arranged.

To fasten the flange upon the corresponding corrugated duct, the end of the flange, provided with one or more longitudinally extending slits, is first form-fitted over the corrugated pipe and then the slits are welded so that, upon contraction of the weldment with cooling, a tight connection of the corrugated duct with the flange results. The seal is finally effected by a weld seam between an end fact of the corrugated pipe and the flange. This type of flange connection with the corrugated pipe excludes and resists stress and ensures a secure seal.

To relieve mechanical stress of the bellows which may result upon thermal contraction of cold cable and terminal parts, there are provided between the flange additional low-thermal-conductivity abutment elements. The low thermal transfer between the flanges can be ensured by a corresponding geometry of the abutment elements and appropriate dependence of 5 thermal conductivity upon temperatures.

The mutual isolation of all vacuum spaces has the advantages that a leak in any one of the spaces will not bring about a failure of the total vacuum, that the leaks can be localized to enable simple detection by an appropriate monitoring or sensing device, and that partial damage to the terminal need not cause loss of vacuum in the cable.

In a further advantageous embodiment of the invention for avoiding thermally generated stress within the terminal, the deep-cooling or low-temperature zone is provided with spaces between the tube shell of the central duct and the radiation shield in such manner that relative axial displacement of the outer duct wall and the radiation shield is permitted while relative radial displacement and torsional displacement remains restricted.

According to still another feature of the invention at the end of the warm zone, an electrically nonconductive tube or pipe is provided which forms a connection between the interior of the conductor through which the warm coolant flows. The coolant duct (supply or outlet) is maintained externally of the system at ground potential.

DESCRIPTION OF THE DRAWING The above and other objects, features and advantages of the present invention will become more readily apparent from the following description reference being made to the accompanying drawing in which:

FIG. 1 is a vertical axial section, partly in diagrammatic form, through a coaxial cable embodying the present invention and formed with a terminal structure;

FIG. 2 is a detail view of a portion of this terminal; and

FIG. 3 is a further detail view, in axial section and partly broken away, illustrating the extremity of the terminal.

SPECIFIC DESCRIPTION In FIGS. 1 through 3 of the drawing, we show a righthand terminal for a low-temperature cable, the latter being illustrated only diagrammatically, it being understood that a left-hand terminal of identical but mirrorsymmetrical design will generally be provided at the opposite end thereof. The illustration of FIG. 1 shows the transition from the cable to the cool zone of the terminal as well as the low-temperature zone or deepcooling zone thereof.

The cable comprises a central or first corrugated duct 1 defining a central chamber la, a coaxial intermediate corrugated duct 2 defining an evacuated second chamber 2a between its inner wall and the outer wall of corrugated duct 1, and a third corrugated duct 3 coaxially surrounding the corrugated duct 2 and defining therewith a further coaxially extending annular chamber 3a through which liquid nitrogen is circulated or in which liquid nitrogen is maintained.

The corrugated duct 3 is coaxially surrounded by an outer corrugated duct 4 and defined with the latter'an annular axially extending chamber 4a which is likewise evacuated.

All of the corrugated ducts are provided at their axially offset (spaced) ends with flanges. Thus, the corrugated duct 4 is provided with a large transverse flange 5 which is connected by axially extending bolts (not shown) to a ring 5a via an O-ring seal 5b received within V-section recesses 5d and 5e of confronting faces of the flange 5 and the ring 5a. The ring 5a is welded to a sheet steel cylinder 5f forming a housing for the remaining flanges and the junction between the cable and the terminal at the low-pressure zone. A transverse cylindrical fitting 5g is welded onto and communicates with the cylinder 5f at a junction 5f and is formed with a flange ring 5h connected by bolts 51' to a plate Sj. An O-ring 5k is received within annular recesses 5m and 5n respectively formed in the ring 5h and the plate 5j. A conduit 28, connected to a suction source as will be described in greater detail hereinafter, is sealed in the plate Sj and communicates with the space 5p within the housing 5f and 5g. At its other axial end, the cylinder 5f is formed with a flange ring Sq which may be connected by axially extending bolts, not shown, to a flange 17a of a shell 17 described in greater detail below. The O-ring Sr is disposed between the annular recess 5s and 17b of the ring Sq and the flange 17a, respectively.

Axially spaced from the flange 5 but likewise extending transversely to the axis of the cable, is a flange 6 which is anchored to the right-hand end of the corrugated pipe 3. This flange 6 is formed with a cylindrical portion 6a within the space 5p and is welded to a cylindrical member 6b defining an annular space 6c communicating with the chamber 3a between the duct 3 and the duct 2. A conduit 11, welded into the cylinder 6b, communicates with the space 60 as well be described in greater detail hereinafter. At its other end the cylinder 6b is received within a recess 7a formed in a flange 7 anchored to the right-hand end of corrugated duct 2 and hence axially spaced from and of smaller diameter than the flange 6.

The flange 7 is provided with a cylindrical portion 7b which surrounds a space 70 communicating with the chamber 2a and welded at its right-hand end to a flange portion 7d connected by a metal bellows 10 to a flange 8 mounted on the right-hand end of the innermost corrugated duct 1. The flange 8 is, consequently, axially offset from the flange 7. A conduit 27 communicates with the space 7c.

From the foregoing, it will be apparent that the corrugated duct 4 is provided with a flange 5, the corrugated duct 3 is provided with a flange 6 the corrugated duct 2 is provided with the flange 7 and the corrugated duct 1 is provided with the flange 8.

Each of the flanges, as shown for the flange 8 in FIG. 1, is provided with a generally cylindrical sleeve portion 8a which is internally corrugated at 8b to conform to the corrugation of the respective duct 1. For fastening the flanges of the respective corrugated ducts, the side of the flanges turned toward the cable C is axially split, i.e., the sleeve portions etc. are longitudinally slit and mounted upon the duct in form-fitting relationship with the corrugations thereof. In other words, each flange and its respective cylindrical portion is then pressed around the duct. The longitudinal slits are then welded shut so that, upon contraction of the weldment or upon cooling a rigid connection is formed between each duct and the respective flange. The final seal between the duct and the flange is formed by welding both parts together along the surface of the flank opposite that from which the cable extends. In the case of duct 1, the flange 8 is welded to it along its right-hand surface at 80 in an annular weld seam.

The sealing of the vacuum chambers against the vacuum space of the terminal is effected by the bellows 9 and 10 which bridge the flanges and 6 of ducts 4 and 3 and the flanges 7 and 8 of ducts 2 and 1, respectively. These bellows, which are composed of material of very low thermal conductivity (e.g. synthetic-resin materials) which retain their flexibility at low temperatures, and relatively small cross-sections and are highly elastic so that heat conduction through them between the internally connected flanges, which may at different temperatures, is maintained.

The bellows 9, for example, is anchored at one axial end to a cylindrical projection 9a on the face of the flange 5 turned away from the cable C and turned toward the terminal T while its other axial end is anchored at 9b to the outer periphery of the flange 6. Similarly bellows 10 is anchored at one axial end 10a to a cylindrical region of the flange 8 extending axially toward the cable C and to a cylindrical portion 1012 extending axially from ring 7d.

All of the mutually parallel flanges are provided with abutment elements 12 (shown by way of example for the flanges 5 and 6) which are of planar or curved configuration and are in the form of triangles whereby the base of the triangle lies against the colder flange while the point lies against the warmer flange. The abutment elements restrict relative torsional, radial or axial movement within the cable. Such a geometrical form of the abutment element is desirable because, although the abutment element may be thermally conductive, the configuration described restricts heat transfer by the flanges bridged by the abutment element.

Liquid nitrogen is fed to the second annular chamber 3a via the nitrogen conduit 11 mentioned previously.

The flange 8 is connected with a duct or shell 13 which is closed by the central passage 13a which widens toward this region. This central passage 13a communicates with a heliumsupply conduit 14 which opens axially in the flange 8 so that deep-cooling helium can be introducted at its usual pressure of, for example, 10 atmospheres absolute. A further tubular sheath 17 coaxially surrounds the duct 13 and defines an annular vacuum space 18 therewith. As described previously, the sheath 17 is sealed to the flange 17a of the fourth corrugated duct 4.

Within the vacuum space 18, an additional thermalradiation shield 19 is provided in spaced relationship to the tubular shells 13 and 17, the radiation shield being connected in a thermally conductive manner with the nitrogen-cooled flange 7 of the nitrogen chamber of the cable. More particularly, the radiation shield 19 is welded to a cylindrical wall 19a which terminates at its left-hand end in a frustoconical metal member 19b which is welded, in turn, to the flange 7 previously described. An opening 190 is provided in the cylinder 19a to permit the conduit 14 to pass therethrough and, to serve as a radiation shield for this conduit 14, a cylindrical standpipe 19d coaxially surrounds the conduit when it emerges through the opening 19c. Members 19a and 1% thus form a thermally conductive link between radiation shield 19 and the cold flange 7.

Since the radiation shield 19 is cooled conductively, i.e., because it is in heat-conducting relation with the nitrogen-cooled flange 7, by nitrogen in the manner described, a separate nitrogen-cooled space in the terminal T need not be provided.

The radiation shield 19 is connected by spacers 20 with the flange 8 so that relative axial movement of the radiation shield and shell 13 is permitted although relative radial and torsional movements are precluded. To this end, the spacer 20 comprises a radially extending finger 20a which reaches into an axially extending slot 20b in which the finger 20a is guided. Since the finger 20a bears along the inner base of the cylinder 19a and a plurality of such fingers may be equispaced about the axis of the device, relative radial displacement of the flange 8 and the radiation shield 19 is precluded. Since the slots 20b confine the finger(s) to axial movement, relative torsional movements of the flange and the radiation shield are precluded. To this extent, in the radial and in the peripheral directions, the shell 13 is rigid with the radiation shield 19.

The conductor 15 of the cable, which is composed of electrically conductive bands electrically wound upon a fluid-permeable synthetic-resin core (here composed of axially spaced synthetic-resin rings 14 having electrical peripheral slot receiving the bands), are disposed within the central duct 1 and extend through the innermost (central) chamber 1a being electrically insulated from the duct 1 by a multiplicity of layers of electrical insulation 16.

The seat formed by the tubular or helically wound layers of insulation is represented at 16. It will be understood that a multi-layer sheath of terminal insulation is disposed in the outer annular compartment 18, this sheath being composed of alternating layers of reflective (metal foil) and fibrous (mineral fibers) material capable of acting as a radiation and convection barrier within this evacuated space.

Within the terminal T the conductor 15 extends coaxially through the widened canal 51 and is at its end connected with the conductors of the cool zone as has been illustrated in FIG. 2.

The insulation 16 only partly covers the conductor 15 in the central chamber. A portion of the deepcooling helium passes through the core of conductor 15 in the uncovered spaces between the turns of the helices and the bottom the rings 14 in the direction of the arrows A shown in FIG. 1. The balance system deepcooling helium flows, as can best be seen from FIG. 2, into the cool zone and serves to maintain the conductors 30 therein at the helium temperature.

A pressure-resisting tube 21 of electrically insulating material (eg fiber glass reinforced synthetic resin) opens into the central channel of the deep-cooling zone and extends from the warm zone through the cool zone. This tube 21 defines a narrow annular clearance 22 with the shell 13 forming the central canal within the terminal.

The outer surface of tube 21 is provided with an electrically conductive conical metal ring 52 maintained at greater potential by a wire coil 52a (inducing spiral flow of coolant) connected to the flange 8. The ring is seated on a step 52b at the cold end of the tube. Opening's 24 in this ring 52 permit helium to pass into the space 51 (FIG. 1) within the tube 21 and around the conductor 15.

A metal ring 25, fitting snugly within the tube 21, has a conical portion bearing upon the insulation 16 and serving to support tube 21. This latter ring, which may be at the-conductor potential, is formed with openings 24a through which the helium may pass. Since the rings 25 and 52 have the indicated electrical potentials, peaks in the electrical field intensity are eliminated in this transition region.

All of the vacuum spaces of the system except for the vacuum space 18 of the deep-cooling and cold zones, are separated from one another and are evacuated by respective pipes 26, 27 and 28 independently of one another so that, upon development of a leak in one of the compartments of the cable or in the compartment 18, the total vacuum of the system will not be broken. The latter pipes themselves are preferably provided, where they pass through openings in the various housing portions, with elastic bellows 29 to permit relative expansion and contraction of the parts without breaking the mermetic seals of the several compartments.

FIG. 2, as noted, represents the portion of the terminal to the right of the part illustrated in FIG. 1, i.e., the terminal portion proximal to the warm zone and generally consisting of the cool zone thereof.

One of the key features of the present invention is that the deep-cooling, cool and warm zones all have respective conductor means electrically connected to them so that the current flows through them in succession. In the deep-cooling zone, the conductor means also the electrical bands forming the conductor 15, in sulated by the jacket 16 and traversed by the helium flows into the cable at least in part. In the cool zone, however, the conductor means consists of an array of axially extending conductive tubes 30 which, at their cold ends, are soldered into an electrically conductive tube sheet 31 in mutually spaced parallel relationship. The tubes 30 may be composed entirely of conductive material (e.g. metal) or may have nonconductive cores surrounded by sheaths of conductive material. In any event, the conductive material forming the tubes 30 has a specific resistance in the operating temperature range T which increases with T where n is greater than 0.5 as noted previously. T is, of course, the absolute temperature in the operating range.

The conductor 15 (i.e., the helical bands) are electrically connected, preferably soldered, to this tube sheet 31 via a metal stub 31a At the warm end, the conductor tube 30 (of the ma terial described e.g. high-purity copper) are soldered directly into bores 36a formed in an end of a thick-wall copper tube 36 constituting the warm zone conductor means. The axial passages 36b communicate between the interiors of the conductor tubes 30 and radial blind bores 32 which, in turn, open into the central bore 360 of conductor 36, thereby venting the portion of the helium traversing this conductor at the free axial end thereof from the terminal.

All of the conductor tubes 30 are surrounded by a common sleeve hermetically sealed at its cold end to the tube seat 31 and its warm end to the conductor 36. For this purpose, an electrically nonconductive ring 34 is provided between the conductor 36 and the sleeve 33 so that the latter does not constitute a conducting path parallel to the conductor tubes 30.

To prevent thermal stresses from arising, a hermetic junction e.g. bellows 35 is provided between the cold side of the tube sheet and the sleeve 33.

Deep-cooling helium thus can flow through the central canal of the deep-cooling zone into the conductor tubes 30 to cool the latter. The warmed helium is discharged through the passage 36a of the conductor 36. The electrical current flows from conductor 36 of the warm zone through the conductor tubes 30 of the cool zone and the tube seat 31 to the cable conductor 15 or vice versa. The space between the tubes 30 is either evacuated or filled with helium at constant pressure.

To separate the cool zone from the warm zone of the terminal, I provide a flange connection substantially at the junction, the flange connection comprising a flange 40 welded to the end of the shell 17 and provided with an axially open V-section groove receiving an O-ring 45. The flange 40 is bolted to the flange 39 formed on fixed to the shell 13 at the warm end thereof. A further seal 46, in the form of an O-ring wedged inwardly by a packing-cone recess or flange 40 bears against a ring 46a surrounding the insulating tube 21 to seal the helium passage.

Between the seals 45 and 46 there is provided an annular space 41 which is vented to the atmosphere by a passage 42. The 'seal 45 thus separates the vacuum space enclosed by shell 17 from the vented space 41. The seals both serve to isolate the oil-filled space 38 of the warm zone from the vacuum and helium spaces.

A porcelain insulator 37, which may extend partly around the cool zone and is sealingly connected to the outer shell 17 via a flange arrangement, is clamped by fingers 44 on bolts 44a via cushions 44b against a seal 44c and the flange 40.

The above described arrangement has the advantage that upon a breakdown at seal 46, helium under relatively high pressure cannot pass directly into the evacuated space 18 to eliminate the effectiveness of this vacuum, the helium being vented at 42. A breakdown of this type, therefore, only results in helium losses. On the other hand, the seal 45 prevents leakage of air into the vacuum space while the two seals together prevent application of the full pressure differential between high pressure helium and the low pressure vacuum from being applied to any one seal.

FIG. 3 shows the Warm zone of the terminal according to the present invention and may be considered an extension of FIG. 2 to the right.

Gaseous warm helium can be recovered from the passage 36c of the conductor 36 by a flexible hose or tube and returned directly to the helium cooling station. The latter may be at ground potential with the flexible tube serving to prevent shunting of electrical current to ground by the helium-recirculating tube.

The pressurizable insulating tube 21 ends in the warm zone and is sealed at 43 to the conductor 36. Over a part of its length in the warm zone the tube 21 may be enclosed in another multi-layer electrically insulating sheath 49. Porcelain insulator 37 extends the full length of the warm zone and encloses the oil filled space 38 defined around the conductor 36. Since the insulator tube 21 extends through all three zones, it must be capable of withstanding a temperature gradient substantially from ambient temperature to the deep-cooling temperature and pressures ranging from ambient pressure to 10 atm. absolute, corresponding to the pressures of the coolant. The end 36d of conductor 36, emerging from the insulator 37, enables the conductor to be connected to a current supply source or to a load.

I claim:

1. A low-temperature system for the transmission of electrical energy comprising a cable having at least three coaxial corrugated ducts defining at least two annular compartments and a central compartment, a conductor extending through said central compartment, and a layer of electrical insulation disposed between said conductor and the wall of the innermost duct, a terminal disposed at one end of said ducts and defining a deep-cooling zone adjacent said ducts, a cool zone adjacent said deep-cooling zone and a warm zone adjacent said cool zone, the improvement wherein said terminal comprises:

respective conductor means electrically connected to said conductor and extending through said zones for transmission of electric current in series through the respective conductor means, said conductor means of said cool zone including an electrically conductive tube;

A sheath of electrical insulating material surrounding said conductor means at least in said deep-cooling zone and defining an axial passage extending along said conductor means in said deep-cooling zone; and 4 means including a conduit held at ground potential communicating with said passage for conducting a cooling medium through the innermost duct around said conductor means through said deepcooling zone and then through said electrically conductive tube.

2. The improvement defined in claim 1, further comprising means defining a common vacuum chamber extending axially along and surrounding said deepcooling and cool zones.

3. The improvement defined in claim 2, further comprising an annular axially extending radiation shield disposed in said chamber, said cable having a coolantcooled radiation shield connected in heat-conducting relation to the radiation shield in said chamber.

4. The improvement defined in claim 2 wherein said chamber is at least partly filled with multilayer thermal insulation.

5. The improvement defined in claim 1 wherein said tube is composed of a conductive material having a specific resistance in the region of the operating temperature T which increases approximately with T", wherein n is greater than 0.5.

6. The improvement defined in claim 1 wherein said conductor means in said cool zone comprises a plurality of such tubes in mutually spaced parallel relationship.

7. The improvement defined in claim 6, further comprising a common shell extending axially along and surrounding said tubes while being insulated electrically at least in part from said tubes.

8. The improvement defined in claim 6 wherein said conductor means of said warm zone comprises a thickwall copper tube having a central channel and formed at its end turned toward said cool zone with a plurality of bores each receiving one of said tubes, said thickwall copper tube being provided with passages connecting said bores with said channel for conducting the coolant from said tubes of said cool zone through said thick-wall copper tube.

9. The improvement defined in claim 6 wherein said tubes are mounted in a common tube sheet, said tubes being surrounded by a common shell connected at the cold end of said cool zone to said tube sheet by an elastic bellows.

10. The improvement defined in claim 6 wherein said tubes are surrounded by a common shell, the space within said shell being evacuated.

11. The improvement defined in claim 6 wherein said tubes are surrounded by a common shell defining around said tubes a compartment under constant coolant pressure.

12. The improvement defined in claim 1 wherein said sheath of electrical insulating material comprises a pressure-sustaining synthetic-resin insulator tube extending axially around said conductor means from said deep-cooling zone through said cool zone and into said warm zone.

13. The improvement defined in claim 12, further comprising a flange connection terminating said cool zone and connecting said warm zone thereto, said flange connection comprising a first annular flange connected to an outer casing enclosing said cool zone, a second flange juxtaposed with said first flange and connected to an inner shell surrounding said insulator tube and defining within said outer casing an evacuated space, a first annular seal between said flanges for sealing said space, a second annular seal between said flanges for sealing the periphery of said insulator tube, and vent means formed between said annular space, at least one of the flanges being mounted upon said insulator tube.

14. The improvement defined in claim 13 wherein said vent means includes an annular chamber formed in one of said flanges between said seals.

15. The improvement defined in claim 12 wherein said warm zone is formed with a porcelain insulator housing surrounding the conductor means of said warm zone and defining therewith an oil-filled space, said conductor means of said warm zone being formed as a copper tube surrounded by said insulator tube and sealed thereto at the end of said insulator tube.

16. The improvement defined in claim 15 wherein said housing extends over a portion of said cool zone and is sealed to a casing thereof defining a vacuum space around said cool zone.

17. The improvement defined in claim 1, further comprising means for evacuating at least one of said annular compartments and including an elastic bellows with high thermal resistance hermetically sealing the end of at least one of the ducts defining the evacuated annular compartment.

18. The improvement defined in claim 17 wherein the ends of the two ducts defining the evacuated annular compartment are provided with mutually offset flanges and said bellows bridges said flanges.

19. The improvement defined in claim 18 wherein said flanges each have an inner portion contoured to fit complementarily to the respective corrugated duct and shrunk thereon upon the cooling of a weld seam formed between two split parts of the inner portion of each flange, each flange being welded to the duct sealingly along a side of the flange turned away from the cable.

20. The improvement defined in claim 17, further comprising triangular abutment elements disposed between flanges at different temperatures and extending generally in an axial direction, said abutment elements having curved bases lying against the relatively colder flange and points lying against the relatively warmer flange.

ment of said casing and said radiation shield but permitting relative axial movement thereof.

22. The improvement defined in claim 1 wherein said passage is formed with means inducing said coolant along a helical flow path upon traversal of the passage.

23. The improvement defined in claim 1 wherein at least one of the walls defining said passage is stepped. 

1. A low-temperature system for the transmission of electrical energy comprising a cable having at least three coaxial corrugated ducts defining at least two annular compartments and a central compartment, a conductor extending throuGh said central compartment, and a layer of electrical insulation disposed between said conductor and the wall of the innermost duct, a terminal disposed at one end of said ducts and defining a deepcooling zone adjacent said ducts, a cool zone adjacent said deepcooling zone and a warm zone adjacent said cool zone, the improvement wherein said terminal comprises: respective conductor means electrically connected to said conductor and extending through said zones for transmission of electric current in series through the respective conductor means, said conductor means of said cool zone including an electrically conductive tube; A sheath of electrical insulating material surrounding said conductor means at least in said deep-cooling zone and defining an axial passage extending along said conductor means in said deep-cooling zone; and means including a conduit held at ground potential communicating with said passage for conducting a cooling medium through the innermost duct around said conductor means through said deepcooling zone and then through said electrically conductive tube.
 2. The improvement defined in claim 1, further comprising means defining a common vacuum chamber extending axially along and surrounding said deep-cooling and cool zones.
 3. The improvement defined in claim 2, further comprising an annular axially extending radiation shield disposed in said chamber, said cable having a coolant-cooled radiation shield connected in heat-conducting relation to the radiation shield in said chamber.
 4. The improvement defined in claim 2 wherein said chamber is at least partly filled with multilayer thermal insulation.
 5. The improvement defined in claim 1 wherein said tube is composed of a conductive material having a specific resistance in the region of the operating temperature T which increases approximately with Tn, wherein n is greater than 0.5.
 6. The improvement defined in claim 1 wherein said conductor means in said cool zone comprises a plurality of such tubes in mutually spaced parallel relationship.
 7. The improvement defined in claim 6, further comprising a common shell extending axially along and surrounding said tubes while being insulated electrically at least in part from said tubes.
 8. The improvement defined in claim 6 wherein said conductor means of said warm zone comprises a thick-wall copper tube having a central channel and formed at its end turned toward said cool zone with a plurality of bores each receiving one of said tubes, said thick-wall copper tube being provided with passages connecting said bores with said channel for conducting the coolant from said tubes of said cool zone through said thick-wall copper tube.
 9. The improvement defined in claim 6 wherein said tubes are mounted in a common tube sheet, said tubes being surrounded by a common shell connected at the cold end of said cool zone to said tube sheet by an elastic bellows.
 10. The improvement defined in claim 6 wherein said tubes are surrounded by a common shell, the space within said shell being evacuated.
 11. The improvement defined in claim 6 wherein said tubes are surrounded by a common shell defining around said tubes a compartment under constant coolant pressure.
 12. The improvement defined in claim 1 wherein said sheath of electrical insulating material comprises a pressure-sustaining synthetic-resin insulator tube extending axially around said conductor means from said deep-cooling zone through said cool zone and into said warm zone.
 13. The improvement defined in claim 12, further comprising a flange connection terminating said cool zone and connecting said warm zone thereto, said flange connection comprising a first annular flange connected to an outer casing enclosing said cool zone, a second flange juxtaposed with said first flange and connected to an inner shell surrounding said insulator tube and defining within said outer casing an evacuated space, a first annular seal between Said flanges for sealing said space, a second annular seal between said flanges for sealing the periphery of said insulator tube, and vent means formed between said annular space, at least one of the flanges being mounted upon said insulator tube.
 14. The improvement defined in claim 13 wherein said vent means includes an annular chamber formed in one of said flanges between said seals.
 15. The improvement defined in claim 12 wherein said warm zone is formed with a porcelain insulator housing surrounding the conductor means of said warm zone and defining therewith an oil-filled space, said conductor means of said warm zone being formed as a copper tube surrounded by said insulator tube and sealed thereto at the end of said insulator tube.
 16. The improvement defined in claim 15 wherein said housing extends over a portion of said cool zone and is sealed to a casing thereof defining a vacuum space around said cool zone.
 17. The improvement defined in claim 1, further comprising means for evacuating at least one of said annular compartments and including an elastic bellows with high thermal resistance hermetically sealing the end of at least one of the ducts defining the evacuated annular compartment.
 18. The improvement defined in claim 17 wherein the ends of the two ducts defining the evacuated annular compartment are provided with mutually offset flanges and said bellows bridges said flanges.
 19. The improvement defined in claim 18 wherein said flanges each have an inner portion contoured to fit complementarily to the respective corrugated duct and shrunk thereon upon the cooling of a weld seam formed between two split parts of the inner portion of each flange, each flange being welded to the duct sealingly along a side of the flange turned away from the cable.
 20. The improvement defined in claim 17, further comprising triangular abutment elements disposed between flanges at different temperatures and extending generally in an axial direction, said abutment elements having curved bases lying against the relatively colder flange and points lying against the relatively warmer flange.
 21. The improvement defined in claim 1 wherein said deep-cooling zone is formed with a casing defining a coolant space around said conductor means in said deep-cooling zone, and a shell coaxially surrounding said casing and defining a vacuum space therewith, and a radiation shield coaxially disposed between said casing and said shell, said casing being formed with spacers preventing relative radial or circumferential movement of said casing and said radiation shield but permitting relative axial movement thereof.
 22. The improvement defined in claim 1 wherein said passage is formed with means inducing said coolant along a helical flow path upon traversal of the passage.
 23. The improvement defined in claim 1 wherein at least one of the walls defining said passage is stepped. 